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Article
Volume 4, Number 1
22 January 2003
9002, doi:10.1029/2002GC000380
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society
ISSN: 1525-2027
Blueschist preservation in a retrograded, high-pressure,
low-temperature metamorphic terrane, Tinos, Greece:
Implications for fluid flow paths in subduction zones
Christopher M. Breeding and Jay J. Ague
Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109,
USA (christopher.breeding@yale.edu; jay.ague@yale.edu)
Michael Bröcker
Institut für Mineralogie 48149, Münster, Germany (brocker@nwz.uni-muenster.de)
Edward W. Bolton
Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109,
USA (edward.bolton@yale.edu)
[1] The preservation of high-pressure, low-temperature (HP-LT) mineral assemblages adjacent to marble
unit contacts on the Cycladic island of Tinos in Greece was investigated using a new type of digital outcrop
mapping and numerical modeling of metamorphic fluid infiltration. Mineral assemblage distributions in a
large blueschist outcrop, adjacent to the basal contact of a 150-meter thick marble horizon, were mapped at
centimeter-scale resolution onto digital photographs using a belt-worn computer and graphics editing
software. Digital mapping reveals that while most HP-LT rocks in the outcrop were pervasively
retrograded to greenschist facies, the marble-blueschist contact zone underwent an even more intense
retrogression. Preservation of HP-LT mineral assemblages was mainly restricted to a 10–15 meter zone (or
enclave) adjacent to the intensely retrograded lithologic contact. The degree and distribution of the
retrograde overprint suggests that pervasively infiltrating fluids were channelized into the marbleblueschist contact and associated veins and flowed around the preserved HP-LT enclave. Numerical
modeling of Darcian flow, based on the field observations, suggests that near the marble horizon,
deflections in fluid flow paths caused by flow channelization along the high-permeability marbleblueschist contact zone likely resulted in very large fluid fluxes along the lithologic contact and
significantly smaller fluxes (as much as 8 times smaller than the input flux) within the narrow, low-flux
regions where HP-LT minerals were preserved adjacent to the contact. Our results indicate that lithologic
contacts are important conduits for metamorphic fluid flow in subduction zones. Channelization of
retrograde fluids into these discrete flow conduits played a critical role in the preservation of HP-LT
assemblages.
Components: 31,303 words, 3 figures.
Keywords: Contact-parallel fluid flow; retrograde; subduction zone; marble; Cyclades; Greece.
Index Terms: 3660 Mineralogy and Petrology: Metamorphic petrology; 8045 Structural Geology: Role of fluids; 8150
Tectonophysics: Evolution of the Earth: Plate boundary—general (3040).
Received 15 May 2002; Revised 15 October 2002; Accepted 18 October 2002; Published 22 January 2003.
Copyright 2003 by the American Geophysical Union
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Breeding, C. M., J. J. Ague, M. Bröcker, and E. W. Bolton, Blueschist preservation in a retrograded, high-pressure, lowtemperature metamorphic terrane, Tinos, Greece: Implications for fluid flow paths in subduction zones, Geochem. Geophys.
Geosyst., 4(1), 9002, doi:10.1029/2002GC000380, 2003.
————————————
Theme: Trench to Subarc: Diagenetic and Metamorphic Mass Flux in Subduction Zones
Guest Editors: Gray Bebout, Jonathan Martin, and Tim Elliott
1. Introduction
[2] Regional metamorphic fluid flow has been well
documented in accretionary terranes as well as in
the deepest levels of subduction zones where arc
magmas are generated [e.g., Noll et al., 1996;
Ague, 1997; Manning, 1997; Breeding and Ague,
2002]. However, for intermediate depths of subduction involving high-pressure, low temperature
(HP-LT) rocks, fluid fluxes and flow paths associated with both prograde and retrograde metamorphic fluid flow remain controversial. Thermal
models and devolatilization reactions suggest that
fluids are produced by slab dehydration and HP-LT
prograde metamorphism along the entire depth
range of subduction [Peacock, 1990]; thus it seems
likely that significant quantities of fluid must have
moved through HP-LT metamorphic terranes at
some time during subduction and/or exhumation.
Most studies of HP-LT crustal rocks from the Alps
and the Cycladic Archipelago of Greece indicate
little evidence for regional-scale prograde fluid
flow [e.g., Philippot and Selverstone, 1991; Selverstone et al., 1992; Getty and Selverstone, 1994;
Barnicoat and Cartwright, 1995; Ganor et al.,
1996; Putlitz et al., 2000]. However, exhumation
of HP-LT rocks in the Cyclades was commonly
accompanied by a near-pervasive retrogression of
HP-LT assemblages that required the presence of
fluids [e.g., Bröcker and Franz, 1998]. The fact that
HP-LT mineral assemblages are preserved at all
suggests that retrograde fluid flow was highly
channelized. Possible conduits for regional, retrograde fluid flow in subduction zones include kmscale tectonic structures (such as mélange zones);
extensive, interconnected vein networks; or lithologic contacts, where mechanical weaknesses
between differing lithologies may cause enhanced
permeability and facilitate fluid flow [e.g., Rye et
al., 1976; Bebout and Barton, 1989; Baumgartner
and Ferry, 1991; Ferry, 1994; Ague, 1997; Ferry et
al., 2001]. We have developed a new, field-based,
real-time digital outcrop mapping technique to look
for retrograde fluid flow signatures by examining
the roles of fluid flux and channelized flow paths in
the preservation of HP-LT mineral assemblages
during exhumation and retrogression of a HP-LT
terrane on the island of Tinos, Cycladic Archipelago, Greece. We have tested our interpretations
using a numerical fluid-flow model that incorporates digital field-mapping observations.
2. Geologic Setting
[3] Many of the Cycladic islands of Greece are
dominated by rocks of the Attic-Cycladic Blueschist Belt, a regionally extensive band of deeply
subducted crustal rocks that underwent HP-LT
metamorphism to blueschist and eclogite facies
during Cretaceous to Eocene subduction of the
Apulian microplate beneath Eurasia [Schliestedt
and Matthews, 1987; Okrusch and Bröcker, 1990;
Bröcker and Enders, 1999]. Metamorphic conditions reached 15 ± 3 kbar and 450–550C [e.g.,
Bröcker et al., 1993]. Subsequent Oligocene-Miocene isothermal exhumation resulted in tectonic
juxtaposition of HP-LT crust and assorted overlying
rock types, consisting of amphibolites, greenschists, and unmetamorphosed rocks. Exhumation
was accompanied by retrograde fluid infiltration,
which was nearly pervasive in some regions and
caused widespread greenschist facies overprinting
of HP-LT rocks. Greenschist facies conditions
reached 4 – 7 kbar and 450 – 500C [Schliestedt
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Upper
Unit
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Tinos
m2
Isternia
Upper Unit
Intermediate Unit
Basal Unit
Tinos
Marble horizons
Miocene granitoids town
m2
HP-LT
m1
5 km
Basal
Unit
N
b
Intermediate Unit
m3
37o
Figure 1. (a) Map of Greece and Cyclades (modified from Bröcker et al. [1993]). Cycladic island of Tinos marked
in red. Extent of Attic-Cycladic blueschist belt shown in gray. (b) Geologic map of Tinos [after Melidonis, 1980;
Bröcker et al., 1993]. Exposed rocks consist mostly of HP-LT Intermediate Tectonic Unit (ITU). ITU marble horizons
in dark gray. Contact zone from marble horizon m2 near Isternia shown in detail in Figure 2a. (c) Schematic tectonostratigraphic section for Tinos [after Bröcker and Franz, 1998]. Three tectonic units separated by major faults (dashed
black lines). ITU is major HP-LT unit, but mostly retrograded to greenschist facies. ITU contains three major marble
horizons (m1, m2, m3). HP-LT remnant assemblages (blue, cross-hatched fields) well preserved only adjacent to
marble horizon contacts.
and Matthews, 1987; Okrusch and Bröcker, 1990].
The source for the infiltrating retrograde fluids is
not known, but was likely related to (1) dehydration
of subducting oceanic crust, (2) tectonic underthrusting and prograde metamorphic devolatilization of newly subducting rock packages [Wijbrans
et al., 1993], and/or (3) fluid release from crystallizing magmas. Despite the apparent pervasive
nature of the retrograde fluid infiltration on islands
like Tinos, isolated HP-LT assemblages were pre-
served, often in close proximity to their retrograded
equivalents. Rocks exposed on Tinos have been
divided into three tectonic units (Figure 1b and 1c)
[cf. Melidonis, 1980; Bröcker and Franz, 1998].
The Upper Tectonic Unit (UTU) is of limited
exposure and consists of a 250 m sequence of
serpentinites, metagabbros, ophicalcites, phyllites,
and amphibolites that are interpreted to be a dismembered ophiolite complex [Katzir et al., 1996;
Putlitz et al., 2001]. UTU rocks underwent greens3 of 11
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chist facies metamorphism during tectonic
emplacement and fluid infiltration, but did not
achieve HP-LT conditions. The base of the UTU
is marked by a low-angle normal fault [Avigad and
Garfunkel, 1989]. The Intermediate Tectonic Unit
(ITU) makes up most of Tinos and consists of a
1250 –1800 m sequence of marbles, calcareous
schists, metasediments, metacherts, metatuffs, and
metabasalts that underwent both HP-LT metamorphism and the widespread greenschist facies overprint. Due to the nearly pervasive retrogression of
the ITU on Tinos, HP-LT assemblages are preserved only in a few isolated zones. The Basal
Tectonic Unit (BTU) occurs only in the northwestern corner of Tinos and consists of intercalated
greenschist facies dolomitic limestones and phyllites that were overthrust by the other tectonic units
[Avigad and Garfunkel, 1989]. Because the dolomitic marble-dominated unit lacks diagnostic mineral assemblages, the metamorphic history of the
BTU has been difficult to decipher [Bröcker and
Franz, 1998]. Preliminary work indicating highsilica phengite compositions suggests that the BTU
underwent HP-LT metamorphism (M. Bröcker and
L. Franz, manuscript in preparation, 2002).
[4] The ITU contains three thick marble sequences
that are used as marker horizons. The marbles are
described from the bottom of the sequence up as
m1, m2, and m3 [Melidonis, 1980]. The original
stratigraphic sequence of interlayered marbles and
metasediments appears to be structurally intact
[Bröcker and Franz, 1998]. Most major zones of
HP-LT preservation on Tinos occur adjacent to the
contacts of thick marble horizons (Figure 1c).
Similarly preserved HP-LT rocks are commonly
associated with marble units in the Cyclades, but
the reason for the spatial association remains unresolved. One explanation for the preservation of HPLT rocks adjacent to the upper contact of the Main
Marble complex on Sifnos was the capping effect of
impermeable marble units that prevented the
upward, cross-layer infiltration of retrograde fluids
from overprinting the blueschists [Matthews and
Schliestedt, 1984]. This mechanism, however, is
not applicable for Tinos, as retrograde fluid infiltration was mostly layer-parallel [Bröcker, 1990;
Bröcker and Franz, 1998]. We investigated one of
the best-preserved HP-LT exposures at the base of
the m2 marble horizon near the village of Isternia on
Tinos for digital outcrop mapping analysis and
numerical modeling of the effects of metamorphic
fluid flow on blueschist preservation (Figure 1b).
3. Digital Outcrop Mapping
[5] A new, real-time, digital outcrop mapping
technique was developed to examine mineral
assemblage distributions in outcrops relative to
lithologic, structural, and metasomatic features
while in the field. Using a digital camera, highresolution photographs of an outcrop were taken
and downloaded into a portable, belt-mounted
computer. The outcrop location was geo-referenced
using handheld GPS and annotated into the image
file. The images were loaded into graphics editing
software and juxtaposed to create a digital backgound for the outcrop map (Figure 2a). Geologic
features (i.e., lithologic units, contacts, faults,
veins, etc.) were digitized onto the basemap at
cm-scale resolution and labeled while standing at
the outcrop. Direct examination of the outcrop and
numerous hand samples at the outcrop allowed
precise mapping of the spatial distribution of
metamorphic mineral assemblages onto the digital
basemap. Diagnostic minerals are coarse-grained in
hand sample, facilitating easy field identification of
mineral assemblage zones (Figure 2b) on the basis
of relative abundance of chlorite (greenschist
facies) and glaucophane (blueschist facies) as follows: (1) Zone 1 = chlorite abundant, glaucophane
not visible; (2) Zone 2 = modal% chlorite >
modal% glaucophane; (3) Zone 3 = modal% glaucophane > modal% chlorite; and (4) Zone 4 =
glaucophane abundant, chlorite not visible. Thin
sections for selected samples were prepared and
examined in the laboratory to confirm the field
observations (Figure 2b).
4. Mineral Assemblage Distributions
[6] Digital outcrop mapping of the relatively wellpreserved HP-LT blueschist exposure near Isternia
reveals distinct distributions of HP-LT and retrograde mineral assemblages. The 250-meter-long
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b
Figure 2. (a) Digital photographs of outcrop at base of m2 marble horizon near Isternia. Lithologic contacts drawn
in black. Outcrop preserves structurally coherent section of ITU with no evidence for faulting. Massive marbles of m2
(not seen in photos) occur 5 meters beyond top-left corner of photo. Base of m2 marked by lowermost marble layer.
(b) Digitized mineral assemblage distribution overlay at same scale as Figure 2a. Lithologic contacts from Figure 2a
included for reference. Basal contact zone of m2 marble unit consists of strongly retrograded (greenschist facies;
Zone 1) thin calcite marbles, acidic metavolcanics, and interlayered metasediments. Below m2, massive metabasalt,
interlayered metasediments and thin metabasalts, and felsic metavolcanics variably retrograded. Extent of
retrogression increases outward (Zones 2 – 3) from a small region of relatively unretrograded blueschist facies
rocks preserved 7 m from basal m2 contact (Zone 4). Zone 1 greenschist assemblages reappear 4 m below lowerright corner of photo in Figure 2a. Quartz-calcite-albite veins up to 0.5 m wide with greenschist facies, retrograded
selvages (Zone 1) occur adjacent to contact zone. Red squares indicate samples used for petrographic verification of
mineral assemblages.
roadcut outcrop contains an intact section of the
ITU immediately below the m2 marble horizon
(Figures 2a and 2b). No evidence for faulting was
observed. The base of the m2 horizon is not a sharp
marble-schist contact, but instead is a gradational,
10–15 m wide contact zone consisting of interlay-
ered calcite marbles, metasediments, and felsic
metavolcanic rocks (Figure 2a). The layered contact
zone quickly grades into massive calcite marbles
upsection (not shown in Figure 2a). Quartz-calcitealbite veins up to 0.5 m thick are present in some
parts of the outcrop. Thin marble layers, as well as
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all rock types within the m2 contact zone contain
greenschist facies mineral assemblages that reflect
near complete retrogression. Altered zones adjacent
to the large veins also contain a greenschist facies
assemblage. Below the m2 contact, remnant HP-LT
mineral assemblages in metabasalts and metasediments are variably preserved in a distinct zonation
pattern.
[7] On the basis of field observations and petrographic examination of thin sections in the laboratory, four mineral zones were mapped for the
outcrop with increasing distance from the base of
the m2 marble horizon (Figure 2b). Sparse, remnant grains of glaucophane, often visible only in
thin section, occur in all zones and indicate HP-LT
protoliths for the greenschist overprint. Zone 1 is
found throughout the basal part of m2, along the
contact, and adjacent to large veins and is dominated by a greenschist facies mineral assemblage
consisting of chlorite, calcite, actinolite, quartz,
epidote, albite, phengite, ± titanite, ± pyrite, ± magnetite, ± glaucophane relics (Figure 2b). Intense
albitization and carbonation, in addition to the
greenschist assemblage occurs adjacent to the large
veins. Relict glaucophane grains are rare in the
marble contact zone and adjacent to the large veins,
suggesting nearly complete retrogression. Zone 2 is
defined at 0.5 to 3.5 meters below the contact zone
and is dominated by the greenschist facies overprint, but the protolith blueschist assemblage of
glaucophane, paragonite, and epidote is discernable
(Figure 2b). In this zone, the greenschist facies
assemblage is restricted to foliation-parallel zones,
likely indicating layer-parallel retrograde fluid
infiltration [Bröcker, 1990]. Zone 3 is 3.5–6.5
meters below the contact and contains more of
the HP-LT blueschist assemblage than the overprint mineralogy (Figure 2b). Zone 4 consists of a
relatively fresh HP-LT blueschist assemblage of
glaucophane, epidote, paragonite, garnet, ± quartz,
± titanite, ± rutile, ± pyrite that is found as a small
exposure 7 meters away from the base of m2
(Figure 2b). Greenschist facies overprinting is rare
in this zone except adjacent to a large retrograde
quartz vein. The extent of Zone 4 blueschists is
difficult to discern because the outcrop becomes
obscured in the lower-right corner of the photo in
Figure 2a. However, Zone 1 assemblages re-appear
4 meters below and to the right of the photo.
Most rocks occurring more than 15 meters below
the base of m2 are dominated by Zone 1 assemblages. Interestingly, glaucophane relics are more
common in the Zone 1 rocks distal to the marble
contact, suggesting that retrogression was more
intense within the contact zone than farther away.
Preservation of the HP-LT assemblage was
restricted to a narrow, 10–15 m-wide, zone adjacent to the marble horizon contact. Similar mineral
assemblage distributions occur near several other
marble horizon contacts on Tinos [Bröcker, 1990]
(Figure 1c).
5. Blueschist Preservation
[8] The well-defined mineral assemblage zonation
pattern surrounding the enclave of HP-LT rocks
suggests that retrograde fluids flowed primarily
around these areas, rather than through them.
Superimposed upon the broad mineral zonation
pattern are minor variations that resulted from
fluid flow along highly permeable, small-scale
features like vein sets and metatuff layers (Figures
2a and 2b). Although many of the preserved HPLT rocks in the Isternia outcrop appear to be
within or near massive metabasalts, the intrinsic
permeability contrasts between metabasalts and
adjacent metasediments were not solely responsible for the lack of retrogression, as similar metabasaltic rock suites that occur far from marble
horizons are completely overprinted. The intimate
spatial association with marble units almost certainly had some influence on the preservation of
HP-LT assemblages. Intense retrogression along
marble contacts immediately adjacent to the blueschist enclaves suggests that the contacts served as
high-flow conduits that were more permeable than
the surrounding metasediments and metabasalts.
The field relations and mineral assemblage distributions revealed by digital outcrop mapping suggest that the retrograde fluid flow was less intense
adjacent to the contacts of some marble horizons.
We postulate that regionally flowing, nearly pervasive, layer-parallel, greenschist facies fluids
were drawn into the highly permeable contacts
of the marble units, resulting in smaller fluid
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fluxes in zones adjacent to the marble contacts due
to deflections of flow paths around the adjacent
zones and into the contacts. The relatively ‘‘dry’’
enclaves interacted with significantly smaller volumes of retrograde fluid and were sites of HP-LT
mineral assemblage preservation. Fluid flow along
lithologic contacts, associated veins, and more
permeable metatuff layers resulted in nearly complete retrogression of rocks in the contact zone and
the preservation of relatively unretrograded blueschist facies rocks within a few meters of the
strongly overprinted contacts.
6. Modeling the Flow System
[9] Our interpretations based on field observations
and digital outcrop mapping were tested using a
steady state numerical model of two-dimensional,
upward-directed flow. The model solved the standard Darcy flow equations for steady state fluid flow
with no chemical reaction:
q¼
k
rPf þ rf grZ ;
m
@ frf
@t
¼ 0 ¼ r qrf ;
ð1Þ
ð2Þ
where q is the Darcy flux, k is permeability, m is
fluid viscosity, Pf is fluid pressure, rf is the fluid
density, g is gravitational acceleration, Z is a
reference coordinate oriented parallel to layering
that increases upward, f is porosity, and t is time.
The equations were solved with spectral-transform
methods to increase model resolution using 769
grid points in the horizontal direction and 139
points in the vertical direction [Bolton et al., 1997,
1999]. The flow region was isothermal and
measured 1 km
1 km with no-flux lateral
boundaries. For simplicity, layering was oriented
vertically. In order to model the effects of morepermeable marble contact zones, two narrow,
elongate zones of high permeability (maximum of
1017 m2) were initially defined within the
otherwise uniform flow region with fixed background permeability of 1019 m2 (Figures 3a and
3b). The 1017 1019 m2 permeability range is
considered reasonable for deep metamorphic settings [Hanson, 1997; Manning and Ingebritsen,
1999]. The sides and edges of each high-permeability zone were nearly step-like in nature with
half of the increase from background to maximum
permeability occurring within 1.6 m. Permeablity
and porosity were directly linked using the model
of Bolton et al. [1997] and were held constant
throughout the simulations. The permeability range
of 1017 1019 m2 yielded a porosity range of
0.084% (background) to 0.28% (high-permeability
zones). The fluid-pressure gradient was adjusted to
produce a maximum pore velocity of 1 m/yr
through conduits with maximum permeability
(1017 m2). This gradient was above hydrostatic,
but somewhat below lithostatic, and falls within the
range of many of Hanson’s [1997] model results
for regional metamorphism. The fluid-pressure
gradient was maintained using constant flux
boundaries at the bottom (input) and top (output)
of the flow region. Input fluid density was 1 g/cm3,
dynamic fluid viscosity was 103 kg/(m.s), and the
fluid was considered incompressible. Other reasonable permeability and porosity scalings and fluid
pressure gradients do not significantly change the
overall flow patterns described below and in
Figures 3a–3c.
[ 10 ] Representative boundary conditions were
chosen for the HP-LT outcrop near Isternia primarily based on field data. Avigad et al. [1988] report
that m2 is 150 m thick in northwestern Tinos, so
the spacing between centers of high-permeability
zones was defined as 150 m. The actual average
permeability within the marble unit was not
known, so the background value was assumed for
the area between the high-permeability zones. The
structure and width (12m) of the high-permeability
zones were based on field observations of the basal
contact zone of the m2 marble horizon in the
outcrop and include high-permeability contributions from the marble-schist contact zone as well
as the associated veins and metatuff layers (Figures
2a and 2b).
[11] Model results indicate that fluid flow paths
within the flow region are deflected into the contact
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b
c
Figure 3. (a) Two-dimensional enlarged central region of Darcy velocity flow fields (i.e., Darcy flux magnitudes)
and streamlines for steady state, upward-directed fluid flow through 1 km 1 km isothermal flow region. Model
parameters based on field observations from Isternia outcrop in Figure 2a. Two parallel, vertically oriented, 12-m
wide, high-permeability (1017 m2) zones defined within uniform flow region of lower permeability (1019 m2)
to simulate marble unit contact zones. Permeability in channels 38 times higher than background. Fluid flux fixed
(1.8 104 m3/m2/yr) at top and bottom of region to observe only the effects of high-permeability channels on fluid
flow paths and fluxes. Spacing between channels set to 150 m, equivalent to actual thickness of m2 marble horizon
adjacent to digitally mapped outcrop in Figure 2a. Fluxes within channels more than an order of magnitude larger
than input flux. Note that low-flux zones, less than 20 m wide, develop adjacent to contacts of modeled marble unit.
Darcy fluxes in low-flux zones 8 times smaller than input flux. Focusing of fluids into highly permeable contact
zones causes limited flow through adjacent enclaves where HP-LT mineral assemblages likely preserved from
retrogression. (b) Cross sections (Y-Y0) for Darcy flux and permeability at z = 500 m in Figure 3a. Darcy fluxes in
low-flux zones 8 times smaller than input flux. Background permeability of 1019 m2 steeply increases to 1017
m2 in modeled contact zones. (c) Effects of model parameter variations on magnitudes of fluid flux in enclaves
adjacent to high-permeability channels. Each parameter independently varied from base model in Figure 3a.
Drawdown factor (DF) defined in equation (3). Increases in permeability contrast (equation (4)) and/or width of highpermeability contact zones results in smaller fluid fluxes (higher DF values) adjacent to modeled marble contacts.
Conversely, increased spacing between channels (i.e., marble unit thickness) causes slight increases in flux adjacent
to channels (lower DF values). Parameters used for Isternia outcrop base model in Figures 3a and 3b shown as open
red circles.
zones, resulting in limited flow and low fluxes
through the immediately adjacent regions (Figures
3a and 3b). Parameters such as contact zone width
(i.e., width of each high-permeability zone), marble
unit thickness (i.e., spacing between high-perme-
ability zones), and permeability contrast between
the background and the high-permeability contacts
were independently varied in a series of steady
state model runs to assess the effects of each
parameter on the magnitude of fluid flow near
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the contact zones. In order to track changes in
Darcy fluid flux in the low flux zones, a drawdown
factor (DF) was defined:
DF ¼
qinput
;
qmin
ð3Þ
where, qinput is the input flux and qmin is the
minimum flux adjacent to contact zones. Higher
DF values indicate lower fluid fluxes adjacent to
contacts relative to the input flux and DF values
approaching 1 indicate that fluid fluxes near
contacts are approaching input values. The model
permeability contrast between the high-permeability zones and the rest of the flow region was
defined as
kcontrast ¼
k contactzone
;
kbackground
ð4Þ
where kcontrast is the permeability contrast,
kcontact-zone is the variable permeability in contact
zones, and kbackground is the fixed background
permeability. The kcontrast was modified by adjusting the permeability within the contact zones
relative to the fixed background permeability.
Variation of kcontrast through a range of 1 to
38 causes DF values to increase from factors of
1 to 8 (Figure 3c). Thus, higher permeability
contrasts draw more flow into the contact zones
and decrease the flux in regions immediately
adjacent to the zones. Variation of the width of
the high-permeability zones from 12 to 40 meters
likewise causes an increase in DF from factors of
8 to 21 (Figure 3c). Conversely, changes in the
spacing of the contact zones (i.e., variations in
model marble unit thickness) from 50 to 200
meters cause a gradual decrease in DF from factors
of 12 to 8 (Figure 3c). Maximum DF values
and the smallest relative fluid fluxes adjacent to
contact zones are achieved with maximum permeability contrast, wide contact zones, and narrow
marble units.
[12] The modeling results are consistent with the
field observations and interpretations. Low-flux
zones in the model are regions where HP-LT
mineral assemblages would most likely be preserved, whereas the rest of the flow system is
characterized by larger fluid fluxes and would have
undergone retrogression (Figure 2b). Our numerical results for the Isternia outcrop suggest that the
permeability contrast must be 20–50, or even
higher, to cause significantly limited fluid fluxes
(i.e., DF factors of 5–10) adjacent to the model
conduits (Figure 3c). Large permeability contrasts
can produce fluid fluxes within the low-flux
regions that are nearly two orders of magnitude
smaller than fluxes within the conduits and as
much as 8 times smaller than the input flux
(Figures 3a–3c). Presumably, these lower fluxes
adjacent to the modeled m2 contact zones resulted
in the preservation of blueschists in the Isternia
outcrop seen in Figure 2b. The model flow system
strongly suggests that the observed preservation of
HP-LT mineral assemblages in 10–15 m enclaves
adjacent to contacts of thick marble horizons was
the result of channelization of fluid flow along
more permeable marble unit contacts. Contactparallel fluid flow not only influences the distribution and preservation of mineral assemblages in
metamorphic terranes, but may also have important
implications for regional fluid flow paths in subduction zones.
7. Conclusions and Implications
[13] Fluid flow paths in subduction zones have
long been the source of debate. Field evidence is
abundant that metamorphic fluids interacted with
HP-LT metamorphosed crust both during subduction and exhumation in Greece. However, most
isotopic and petrologic studies indicate no evidence
for large-scale fluid flow in the Cyclades, leading
to suggestions that flow must have been limited or
extremely channelized. Pervasive retrogression of
regionally extensive rock packages, as observed on
Tinos, requires large-scale fluid flow during exhumation, yet until now, no regional-scale retrograde
fluid conduits had been identified. Digital outcrop
mapping of mineral assemblages in the HP-LT
rocks of Tinos and numerical modeling of the flow
system indicate that some lithologic contacts and
associated veins served as major conduits for
regionally moving fluids. The HP-LT terrane
exposed on Tinos underwent near pervasive retrograde fluid infiltration during exhumation. Con9 of 11
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tacts between large marble horizons and surrounding metasedimentary and metaigneous rocks were
highly retrograded and commonly fractured, and
are inferred to have been zones of elevated permeability. We conclude that fluids were preferentially
channelized into these zones of high permeability
and high fluid flux. As a consequence, HP-LT
assemblages were preserved in the directly adjacent low-flux regions. Evidence for a significant,
contact-parallel component of metamorphic fluid
flow provides important insight into the nature of
fluid flow pathways at intermediate depths in
subduction zones. The reported absence of stable
isotopic fluid alteration signatures (either prograde
or retrograde) in most HP-LT terranes may be the
result of localization of regionally flowing fluids in
a layer-parallel/fracture flow system within the
subducting slab and downgoing continental crust.
Our digital mapping and modeling strongly suggest that lithologic contacts and associated fractures/veins are one of the elusive, major flow
channels for retrograde metamorphic fluids in
subduction zones. This field-based identification
of flow conduits in the Attic-Cycladic Blueschist
Belt provides some measure of reconciliation
between the hitherto contrasting views of fluid
flow during subduction, metamorphism, and exhumation of HP-LT terranes.
Acknowledgments
[14] We thank Elizabeth Donald for field assistance and the
Greece Institute of Geology and Mineral Exploration (IGME)
for logistical support. Constructive reviews by John M. Ferry,
C. Page Chamberlain, and Gray E. Bebout significantly
improved the manuscript. Financial support from National
Science Foundation grants EAR-0105927, EAR-9727134,
and United States Department of Energy grant DE-FG0201ER15216 is gratefully acknowledged.
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